Apparatus for bending a steel plate

ABSTRACT

An apparatus for bending a steel plate to shape the same into any desired configuration having a compound curved surface, characterized in that said apparatus comprises an upper punch group consisting of at least three rows of punches aligning in the widthwise direction of the steel plate which punch group is disposed above said steel plate and which is adapted to form a curved surface suitable for shaping said steel plate into said desired configuration, and a lower punch group consisting of at least three rows of punches aligning in the widthwise direction of the steel plate which punch group is disposed under said steel plate and which is adapted to press said steel plate so as to place it along said upper punch group, said upper punch group being disposed in such manner that the punches therein are individually adjustable in height and the respective punch rows in said at least three rows of punches can be arbitrarily preset in position with respect to the relative heights of the rows and the distances between the rows, said lower punch group being disposed in such manner that the respective punch rows can be arbitrarily preset in position with respect to the distances between the rows, whereby the curvatures of said plate in the widthwise and lengthwise directions may be arbitrarily established.

United States Patent Iwasaki et a1.

[ June 17, 1975 APPARATUS FOR BENDING A STEEL PLATE [75] Inventors: Yasuhiro Iwasaki; Hiroshi Shiota;

Seido Seko, all of Hiroshima, Japan [73] Assignee: Mitsubishi Jukogyo Kabushiki Kaisha, Tokyo, Japan [22] Filed: Dec. 26, 1973 [21] Appl. No.: 428,302

{52] US. Cl. i. 72/385; 72/383; 72/399;

72/403 [51] Int. Cl 821d 13/02 [58] Field of Search 72/380, 381, 382, 383,

Primary Examiner-C. W. Lanham Assistant Examiner.lames R. Duzan Attorney, Agent, or FirmCushman, Darby & Cushman [57] ABSTRACT An apparatus for bending a steel plate to shape the same into any desired configuration having a compound curved surface, characterized in that said apparatus comprises an upper punch group consisting of at least three rows of punches aligning in the widthwise direction of the steel plate which punch group is disposed above said steel plate and which is adapted to form a curved surface suitable for shaping said steel plate into said desired configuration, and a lower punch group consisting of at least three rows of punches aligning in the widthwise direction of the steel plate which punch group is disposed under said steel plate and which is adapted to press said steel plate so as to place it along said upper punch group, said upper punch group being disposed in such manner that the punches therein are individually adjustable in height and the respective punch rows in said at least three rows of punches can be arbitrarily preset in position with respect to the relative heights of the rows and the distances between the rows, said lower punch group being disposed in such manner that the respective punch rows can be arbitrarily preset in position with respect to the distances between the rows, whereby the curvatures of said plate in the widthwise and lengthwise directions may be arbitrarily established.

10 Claims, 7 Drawing Figures SHEET PATENTEDJUN 17 I915 Wl/l'? ii II M rill/Illa W WW 5 APPARATUS FOR BENDING A STEEL PLATE FIELD OF THE INVENTION The present invention relates to a method and apparatus for shaping a steel plate into a compound curved surface or even into a twisted configuration.

BACKGROUND OF THE INVENTION In general, outside plate of a hull at a bow or at a stern have a complex shape, so that steel plates are often shaped not only into a compound curved surface but also into a twisted configuration. Accordingly, it has heretofore been a common practice that after a steel plate has been bent in one direction by means of a press or a bending roll, it is shaped into a desired configuration through a thermal process, that is, by socalled linear heating. However, in case of employing this method, not only there were disadvantages that the working time is long, that a punch and a die corresponding to the curved surface configuration are required, and that a high degree of skill is needed, but also the automation of the working apparatus was extremely difficult.

In view of the above-mentioned various disadvantages in the prior art, the inventors of the present invention proposed a plate-like die system as means for improving the working efficiency in case of bending a great number of steel plates into the same configuration as is the case with a spherical tank shell. In this plate-like die system, plate-like dies having a length equal to the entire width of a steel plate are used, one for the upper die and three for the lower dies, so that an extremely efficient work can be conducted in comparison to the conventional systems employing a spherical head punch and a concave die.

In more particular, according to the plate-like die system, the desired curvature of the steel plate in its widthwise direction is given to the dies, while the desired curvature of the same in the lengthwise direction is established by giving appropriate level differences to the three plate-like dies to be used as the lower dies, and the steel plate is pressed with the plate-like die to be used as the upper die so as to obtain the desired curvatures in the respective directions.

As described, in the plate-like die system, since the curvatures given to the plate-like dies are fixed, this system is not suitable for the diversity in the so-called many-kinds small-amount production as is the case with the manufacture of outside plate of a hull, in which case not only the curvatures of the curved surface of the steel plate are varied depending upon the location where said steel plate is used, but also the curvature is varied point by point even in one steel plate.

SUMMARY OF THE INVENTION The present invention obviates all these disadvantages, and it provides a novel apparatus for bending a steel plate, in which three rows of punches adapted to establish a desired curved surface are provided in each of upper and lower punch groups instead of preparing a punch and a die for each time corresponding to various possible curved surface configurations, as well as a novel method for shaping a steel plate by means of said apparatus.

In other words, in order to give any arbitrary curvatures to a steel plate in its widthwise and lengthwise directions, since bending work is to be carried out, upon establishing said curved surface with punch rows, at first with regard to the lengthwise direction, two rows of punches are provided, for instance, on the lower portion, and between the two punch rows the steel plate could be pressed by a single row of upper punches. Then it is a matter of course that in order to give a curvature in the widthwise direction to the steel plate, said respective punch rows are given with the corvature in the widthwise direction. However, in this case, since some deviation in stroke of the upper punch row may affect upon the bending precision of the shaped article, it is necessary from an industrial point of view that a lower punch row is further provided oppositely to the upper punch row so as to limit the stroke.

However, when it is desired to further give a twist to said curved surface simultaneously, it is necessary to clamp the steel plate with the upper and lower punch rows at the center and to apply twisting action to the steel plate with the upper and lower punch rows on the left and right sides. Therefore, for the purpose of shaping a complexed curved surface as is the case with outside platings ofa hull, at least three rows of punches are required on both the upper and lower portions.

The construction of the subject apparatus is characterized in that upper and lower punch groups each consisting of three rows of punches are arrayed in an opposed relationship in the widthwise direction of a steel plate over a length exceeding the maximum width of the steel plate, the respective punches in said punch groups are individually adjustable in height, and the respective punch rows in said punch groups are disposed in such manner that the relative heights of the rows and the distances between the rows may be arbitrarily established, whereby the curvatures of said steel plate in the widthwise and lengthwise directions may be established in any desired manner.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

In the drawings,

FIG. 1 is a general perspective view of the apparatus for bending a steel plate according to the present invention;

FIG. 2 is a longitudinal cross-section view showing in detail the proximity of the tip end of the punch;

FIG. 3 is a representation of a typical bent configuration of a steel plate given a twist;

FIG. 4 is a schematic representation in an integral form of a curved surface to be formed by the punch rows for giving a twist to a steel plate;

FIG. 5 is a side view of upper and lower punch groups each consisting of three punch rows under the state of pressing a steel plate for shaping it into a twisted configuration;

FIG. 6 is a side view partially in section showing a mechanical method for adjusting the distances between the adjacent punch rows in the punch group consisting of three punch rows; and

FIG. 7 is a vertical section taken along line 7-7 of FIG. 6.

Reference numeral 1 designates a steel plate to be shaped, numeral 2 designates an upper frame of a press body, numeral 3 designates side frames of the same, numeral 4 designates a lower frame, and these frames are rigidly coupled together by means of keys 33 to form the press body.

Reference numeral 5 designates accommodating boxes for the respective punch groups, which are mounted on the upper and lower frames, respectively, and which accommodate hydraulic cylinders or screw boxes 6, 8 and punch groups 7, 9.

Reference numeral 10 designates a feeding or receiving table, and numeral ll designates rollers which are mounted on the table 10.

The punch rows are arrayed in three rows trans versely of the direction of feed of the steel plate both on the upper and lower sides, and along the widthwise direction of the steel plate any desired number of punches are disposed in one punch row. When a steel plate is bent into a desired configuration by means of these punch groups, a curved surface is established by said punch group. It is possible to preliminarily program this curved surface and to establish said curved surface by means of a small size of electronic computer 12. Reference numeral 13 designates an operation panel for the press.

With regard to the individual punches in said punch group, in order to make the tip end of the punch freely direct along the curved configuration of the steel plate as the steel plate is deformed, in each punch a tip end portion 14 is adapted to make contact with a punch stem 16 via a spherical surface, as shown in FIG. 2. For assembling such a structure, the tip end portion 14 is machined in a concave form so as to fit around a spherical portion up to the depth of the maximum diameter, and after it is placed over a spherical head portion of a punch stem 16, a holding plate is coupled to the tip end portion 14 by means of bolts 17.

The extension and retraction of the punches for establishing a curved surface by means of the upper punch group or the lower punch group could be carried out by employing a screw system for both the upper and lower punch groups. However, it is more preferable to construct either the upper or lower punch group in a screw type and to employ a hydraulic system for the opposite side of punch group. More particularly, if both the upper and lower punch groups are constructed in a screw type, the heights of the punches must be controlled in both the upper and lower punch groups for establishing a curved surface. However, if the punch group on one side is constructed so as to be hydraulically operated and the punch stroke is controlled only on the other side where a screw system is employed, then the punches on the hydraulic side need not be controlled so long as they are applied with a sufficient hydraulic pressure to make the steel plate bend along the punch group of screw type on the other side.

One preferred embodiment of the means for adjusting the distance between the adjacent punch rows in the three rows of punches, is shown in FIGS. 6 and 7. These figures show the punch group on the lower side as being operated hydraulically.

On the lower frame 4 are mounted guide members 29 for sliding the punch rows therealong, the punch row at the center is fixed in position, and the punch rows on the opposite sides are moved by means of screws 30 to adjust the distance between the adjacent punch rows in the three rows of punches. Reference numeral 34 designates a gear box. As shown in FIG. 7, at the bottom portion of the punch rows, grooves 31 for slide guide and threaded bores 32 are provided in multiple.

In order to achieve the adjustment of the distance between the punch rows, a hydraulic cylinder and a ram can be employed in place of the screw means.

Naturally, the aforementioned mechanism can be employed in the upper punch rows.

Since the construction of the apparatus according to the present invention is constructed as described above, said apparatus can function as fully described in the following.

In FIG. 1, a steel plate 1 to be worked is fed into the press by means of the steel plate feed rollers 11 on the left or right side.

In the punch section, the heights of the punches and the distances between the adjacent punch rows are preset so as to meet the configuration into which the steel plate 1 is to be bent. For instance, in case that it is intended to bend the steel plate 1 in a downwardly convex shape both in the lengthwise and widthwise directions as shown in FIG. 1, in the upper punch group the heights of the individual punches and the distances between the punch rows are adjusted taking into account the spring-back effect of the steel plate so that the upper punch group may form a downwardly convex surface.

In the lower punch group, if it is constructed in a screw type, it is necessary to adjust the heights of the individual punches and the distances between the punch rows so as to form a surface conformable to the surface of the upper punch group taking into account the thickness of the steel plate. However, if the lower punch group is adapted to be hydraulically operated, then it is necessary to adjust only the distances between the punch rows so as to conform to the upper punch rows.

If the steel plate 1 is to be applied with fixed curvatures in the lengthwise and widthwise directions, then the relative positions of the respective punch rows are kept fixed, and the steel plate 1 is intermittently fed step by step from its one end towards the other end as operated by the operation panel 13 to repeat the press bending operation. Then the desired configuration can be obtained.

If the curvatures which give the desired configuration to the steel plate 1 are not fixed, the necessary bending work of the steel plate can be conducted by changing the heights of the individual punches and the distances between the punch rows for each step of feed of the steel plate 1 (In this case also, in a hydraulically operable punch group only the adjustment of the distances between the punch rows is necessary.) and by repeating the press bending operation.

Now description will be made on the case where the steel plate is to be bent into the configuration which has some curvatures in both the lengthwise and widthwise directions and also which is twisted as shown in FIG. 3.

In this case, the punch rows 18, 19 and 20 must be established as shown in FIG. 4, in which the punch rows are depicted schematically in an integral form. More particularly, although a desired twist can be given to a steel plate by clamping the steel plate with two rows of punches in both the upper and lower punch groups if it is required to simply twist the steel plate, in case that any curvatures are to be given to the steel plate in both the lengthwise and widthwise directions, at least three rows of punches are required and the envelope line of each punch row must be given with a curvature.

Explaining the aforementioned process with reference to FIG. 5 which shows the punch, groups as viewed from one side, the upper punch group consisting of three rows is equipped on the upper frame 2, and the individual punches in the respective punch rows are preset in height as varying in the widthwise direction of the steel plate in such manner that in the left side punch row as viewed in FIG. 5 the foremost punch is highest and the tip ends of the punches become successively lower towards their rearmost end as depicting a curve with their envelope line, that in the right side punch row as viewed in the same figure the foremost punch is lowest oppositely to the left side punch row and their tip ends become successively higher towards their rearmost end, and that the punches in the central row form an intermediate shape of envelope line.

Each of the respective punches in these punch rows has a construction such that a punch tip end portion 22 and a threaded punch stem 21 may be coupled via a spherical seat as explained previously with reference to FIG. 2. The vertical movement of said punch is driven by a motor 25 which is directly connected to the other end of the punch stem 21. If the motor is kept free, only the motor casing will be brought into free rotation, and so, by mounting a rotation constrainer 36 to said motor casing and making said constrainer engage with a rotation constraining jig 27 provided on the inner side wall of the accommodating box 5, the rotation of the motor casing is constrained to move the punch in the vertical direction. In this connection, a female screw to be engaged with the male screw on the punch is threaded in the member at the top of the accommodating box 5.

On the other hand, the lower punch group consisting of three rows and mounted on the lower frame 4 comprises hydraulic cylinders 28 and rams 23, and at the tip end of said ram 23 is mounted a punch tip end portion 24 having the same structure as the punch tip end portion 22. These members are contained within an accommodating box 5'.

Now, if a steel plate is placed on the lower punch rows and a hydraulic pressure is applied to said punch rows, then said steel plate is pressed against the upper punch rows, and so the steel plate can be shaped into the desired compound curved surface having a twist as it is bent perfectly along the curved surface established by the upper punch rows. In addition, since the steel plate can be fed intermittently step by step, even a large size of steel plate can be shaped into a desired configw ration.

In case that the shaped configuration of the steel plate is different depending upon the location on the steel plate, also the steel plate can be shaped into any configuration by carrying out the adjustment of height in the upper punch group as well as the adjustment of distances between the adjacent rows in the three rows of punches for each press bending operation.

Although the punch rows are shown in FIG. 5 as if the distances between the adjacent punch rows are fixed in both the upper and lower punch groups, actually in order to adjust the distances between the adjacent punch rows slide guide members 29 are mounted, for example, to the lower frame, and the left and right punch rows are provided with guide grooves 31 corresponding to the slide guide members 29 to allow said punch rows to move as guided by the slide guide members 29 as shown in FIG. 6, because the central punch row could be fixed in position. For moving these punch rows, threaded bores 32 are provided in the lower portion of the punch rows, and screws 30 are rotated by a motor via a gear box 34 which serves to reduce the rotating speed of the motor. Since the threaded bore 32 and the screw 30 are meshed with each other, the left and right punch rows can be driven in the leftward and rightward directions, respectively. in this connection, while a mechanical driving method has been described above, the so-called hydraulic method using a hydraulic cylinder and a ram, can be equally utilized.

in summary, when the apparatus for bending a steel plate according to the present invention is used, a steel plate can be easily subjected to a bending work to be shaped into any desired configuration of compound curved surface without employing a specifically designed die, and also the automation of the entire apparatus can be readily achieved. Therefore, the contribution of the present invention to the industry is significant.

What is claimed is:

1. Apparatus for bending a steel plate to provide complex curvature thereof, comprising:

means for intermittantly advancing a steel plate along a longitudinal path through a bending station;

a first punch group including a plurality of individual punches disposed of in at least three longitudinally spaced, transversally extending rows at the bending station, all of the punches in the first punch group being oriented for contact with one face of the steel plate to be bent;

a second punch group including a plurality of individual punches disposed in at least three longitudinally spaced, transversally extending rows at the bending station, all of the punches in the second punch group being oriented for contact with the opposite face of the steel plate to be bent, in direct opposition to corresponding ones of the punches in the first punch group;

frame means and means mounting the first and second punch group on the frame means at the bending station;

the mounting means for at least one of the two recited punch groups including an adjustment device between each punch and the frame means, and remotely operable control means for operating each adjustment device to change the proximity of each individual punch to the path of the steel plate through the bending station;

the mounting means for at least the other of the two recited punch groups including means for advancing each punch of said other punch group this is within the width of the increment of the steel plate that is within the bending station into such forceful contact with that increment that said increment is engaged by punches of the one punch group on a first face thereof and by punches of the other punch group on the second face thereof, in opposed pairs, separated by the thickness of the steel plate; and means for retracting the punches of said other punch group once the steel plate of that increment has been bent by said forceful contact,

whereby, the steel plates may be intermittantly advanced by an increment and progressively bent by increments, with the control means being operated to change the proximity of the individual punches of the first punch group to the path of the steel plate through the bending station between each bending operation of the two recited punch groups in which the curvature planned for the next increment of the steel plate to be bent differs from the curvature applied to the steel plate increment just bent.

2. The apparatus of claim 1 wherein the control means includes:

a first means for accepting and storing commands for each adjustment device, equating to whether and how much each adjustment device is to be operated to adjust the proximity of the respective indi vidual punches for each successive increment of bending; and

a second means for withdrawing all said commands for the particular next increment of bending from the first means and for communicating the withdrawn commands to the respective adjustment device for initiating operation thereof for the next increment of bending.

3. The apparatus of claim 1, wherein each adjustment device includes:

remotely operable motor means mounted on the frame and having a reversible rotational output; and

a direction changing device installed between the motor output and the respective punch for translating rotation of the output into proximity adjusting movement of the respective punch.

4. The apparatus of claim 3 wherein the direction changing device comprises a screw and nut arrangement having one part thereof secured to the motor output and the other part thereof secured to the respective punch.

5. The apparatus of claim 1, wherein each advancing and retracting means includes:

a hydraulic piston and cylinder arrangement, and means for pressurizing contained hydraulic fluid to advance and retract the piston with respect to the cylinder, one part of the piston and cylinder arrangement being mounted on the frame and the other part thereof being secured to the respective punch.

6. The apparatus of claim 1, wherein the mounting means of punches of said other punch group are replicates of the mounting means of the punches of said one punch group and wherein each advancing means and adjustment device includes:

remotely operable motor means mounted on the frame and having a reversible rotational output; and

a direction changing device installed between the motor output and the respective punch for translating rotation of the output into proximity adjusting movement of the respective punch.

7. The apparatus of claim 6, wherein the control means includes:

a first means for accepting and storing commands for each adjustment device, equating to whether and how much each advancing means and adjustment device is to be operated to adjust the proximity of the respective individual punches for each successive increment of bending; and

a second means for withdrawing all said commands for the particular next increment of bending from the first means and for communicating the withdrawn advancing means and adjustment devices for initiating operation thereof for the next increment of bending. 8. The apparatus of claim 1 wherein: the frame means includes a fixed portion and a plu- S rality of adjustable portions mounting said punch group mounting means of the longitudinally foremost and the longitudinally rearmost of said three longitudinally spaced rows of punches in both said punch groups thereon for longitudinal movement relative to the middle row of each punch group, whereby the inter-row spacing of the punches may be adjusted between increments of bending.

9. A method for bending steel plate to provide complex curvature thereof, comprising:

a. advancing a steel plate by a first increment along a longitudinal path into a bending station which is confronted from opposite sides by two groups of punches set in at least three opposed rows, spaced longitudinally of the path and extending transversally of the path;

b. before bending the first increment of the steel plate, adjusting the proximity of the individual punches of one of the punch groups to the path of the steel plate through the bending station to provide the punches of said one punch group with a collective topography corresponding to the desired curvature of the first increment of the steel plate;

c. advancing each punch of the other punch group that is within the width of said first increment of the steel plate into such forceful contact with said first increment that said first increment is engaged by punches of the one punch group on a first face thereof and by punches of the other punch group on the second face thereof, in opposed pairs, separated by the thickness of the steel plate;

d. retracting the punches of said other punch group once the steel plate in the first increment has been bent to said desired curvature by said forceful Contact;

e. advancing a succeeding, second increment of the steel plate along said path into the bending station;

f. before bending the second increment of the steel plate, adjusting the proximity of the individual punches of said one punch group to the path of the steel plate through the bending station to provide the punches of said one group with a collective topography altered from that of step (b) and corresponding to the desired curvature of the second increment of the steel plate;

g. advancing each punch of the other punch group that is within the width of said second increment of the steel plate into such forceful contact with said second increment that said second increment is engaged by punches of the one punch group on a first face thereof and by punches of the other punch group on the second face thereof, in opposed pairs, separated by the thickness of the steel plate; and

h. retracting the punches of said other punch group once the steel plate in the second increment has been bent to said desired curvature by said forceful contact.

10. The method of claim 9 including the step, interposed between steps (d) and (g), of changing the inter- (.5 row spacing of the punches of both punch groups. 

1. Apparatus for bending a steel plate to provide complex curvature thereof, comprising: means for intermittantly advancing a steel plate along a longitudinal path through a bending station; a first punch group including a plurality of individual punches disPosed of in at least three longitudinally spaced, transversally extending rows at the bending station, all of the punches in the first punch group being oriented for contact with one face of the steel plate to be bent; a second punch group including a plurality of individual punches disposed in at least three longitudinally spaced, transversally extending rows at the bending station, all of the punches in the second punch group being oriented for contact with the opposite face of the steel plate to be bent, in direct opposition to corresponding ones of the punches in the first punch group; frame means and means mounting the first and second punch group on the frame means at the bending station; the mounting means for at least one of the two recited punch groups including an adjustment device between each punch and the frame means, and remotely operable control means for operating each adjustment device to change the proximity of each individual punch to the path of the steel plate through the bending station; the mounting means for at least the other of the two recited punch groups including means for advancing each punch of said other punch group this is within the width of the increment of the steel plate that is within the bending station into such forceful contact with that increment that said increment is engaged by punches of the one punch group on a first face thereof and by punches of the other punch group on the second face thereof, in opposed pairs, separated by the thickness of the steel plate; and means for retracting the punches of said other punch group once the steel plate of that increment has been bent by said forceful contact, whereby, the steel plates may be intermittantly advanced by an increment and progressively bent by increments, with the control means being operated to change the proximity of the individual punches of the first punch group to the path of the steel plate through the bending station between each bending operation of the two recited punch groups in which the curvature planned for the next increment of the steel plate to be bent differs from the curvature applied to the steel plate increment just bent.
 2. The apparatus of claim 1 wherein the control means includes: a first means for accepting and storing commands for each adjustment device, equating to whether and how much each adjustment device is to be operated to adjust the proximity of the respective individual punches for each successive increment of bending; and a second means for withdrawing all said commands for the particular next increment of bending from the first means and for communicating the withdrawn commands to the respective adjustment device for initiating operation thereof for the next increment of bending.
 3. The apparatus of claim 1, wherein each adjustment device includes: remotely operable motor means mounted on the frame and having a reversible rotational output; and a direction changing device installed between the motor output and the respective punch for translating rotation of the output into proximity adjusting movement of the respective punch.
 4. The apparatus of claim 3 wherein the direction changing device comprises a screw and nut arrangement having one part thereof secured to the motor output and the other part thereof secured to the respective punch.
 5. The apparatus of claim 1, wherein each advancing and retracting means includes: a hydraulic piston and cylinder arrangement, and means for pressurizing contained hydraulic fluid to advance and retract the piston with respect to the cylinder, one part of the piston and cylinder arrangement being mounted on the frame and the other part thereof being secured to the respective punch.
 6. The apparatus of claim 1, wherein the mounting means of punches of said other punch group are replicates of the mounting means of the punches of said one punch group and wherein each advancing means and adjustment device includes: remotely operable motor mEans mounted on the frame and having a reversible rotational output; and a direction changing device installed between the motor output and the respective punch for translating rotation of the output into proximity adjusting movement of the respective punch.
 7. The apparatus of claim 6, wherein the control means includes: a first means for accepting and storing commands for each adjustment device, equating to whether and how much each advancing means and adjustment device is to be operated to adjust the proximity of the respective individual punches for each successive increment of bending; and a second means for withdrawing all said commands for the particular next increment of bending from the first means and for communicating the withdrawn advancing means and adjustment devices for initiating operation thereof for the next increment of bending.
 8. The apparatus of claim 1 wherein: the frame means includes a fixed portion and a plurality of adjustable portions mounting said punch group mounting means of the longitudinally foremost and the longitudinally rearmost of said three longitudinally spaced rows of punches in both said punch groups thereon for longitudinal movement relative to the middle row of each punch group, whereby the inter-row spacing of the punches may be adjusted between increments of bending.
 9. A method for bending steel plate to provide complex curvature thereof, comprising: a. advancing a steel plate by a first increment along a longitudinal path into a bending station which is confronted from opposite sides by two groups of punches set in at least three opposed rows, spaced longitudinally of the path and extending transversally of the path; b. before bending the first increment of the steel plate, adjusting the proximity of the individual punches of one of the punch groups to the path of the steel plate through the bending station to provide the punches of said one punch group with a collective topography corresponding to the desired curvature of the first increment of the steel plate; c. advancing each punch of the other punch group that is within the width of said first increment of the steel plate into such forceful contact with said first increment that said first increment is engaged by punches of the one punch group on a first face thereof and by punches of the other punch group on the second face thereof, in opposed pairs, separated by the thickness of the steel plate; d. retracting the punches of said other punch group once the steel plate in the first increment has been bent to said desired curvature by said forceful contact; e. advancing a succeeding, second increment of the steel plate along said path into the bending station; f. before bending the second increment of the steel plate, adjusting the proximity of the individual punches of said one punch group to the path of the steel plate through the bending station to provide the punches of said one group with a collective topography altered from that of step (b) and corresponding to the desired curvature of the second increment of the steel plate; g. advancing each punch of the other punch group that is within the width of said second increment of the steel plate into such forceful contact with said second increment that said second increment is engaged by punches of the one punch group on a first face thereof and by punches of the other punch group on the second face thereof, in opposed pairs, separated by the thickness of the steel plate; and h. retracting the punches of said other punch group once the steel plate in the second increment has been bent to said desired curvature by said forceful contact.
 10. The method of claim 9 including the step, interposed between steps (d) and (g), of changing the interrow spacing of the punches of both punch groups. 